1. Technical Field
Embodiments described herein relate to a method of manufacturing a resonant transducer.
2. Related Art
FIGS. 6-17 illustrate the configuration of an essential part of a related-art resonant transducer. FIG. 6 illustrates the structure of an essential assembly, FIGS. 7-15 illustrate a manufacturing process, FIGS. 16A and 16B illustrate a circuit corresponding to the essential assembly of FIG. 6, and FIG. 17 illustrates the operation of the related-art resonant transducer.
A manufacturing process will be described below. As shown in FIG. 7, a silicon oxide film 10a is formed on an n-type silicon single crystal substrate 1 and then patterned. A recess is formed in the substrate 1 by undercutting it in an oxide-film removed region, and p+ single crystal silicon 11 is formed there at a boron concentration of 1018 cm−3 by selective epitaxial growth.
Then, p++ single crystal silicon 12a is grown upward on the surface of the p+ single crystal silicon layer 11 at a boron concentration of 3×1019 cm−3 or higher. The p++ single crystal silicon layer 12a will later serve as a vibration beam 3, and a gap will be formed later under the vibration beam 3 in the space currently occupied by the p+ single crystal silicon layer 11.
Then, as shown in FIG. 8, a silicon oxide film 10b is formed on the surface of the substrate 1 including the surface of the p++ single crystal silicon layer 12a. Recesses D formed by removing the silicon oxide film 10b partially will later serve as grounding portions of a shell 4 to the substrate 1.
Then, as shown in FIG. 9, a silicon nitride film 13 is formed on the surface of the silicon oxide film 10b as well as inside the recesses D and then patterned. A gap will be formed later over the vibration beam 3 in the space that is currently occupied by those portions of the silicon oxide film 10b and the silicon nitride film 13 which are located on the p++ single crystal silicon layer 12a (vibration beam 3). The capacitance is determined by the thicknesses of these films and the area of the vibration beam 3. Therefore, the capacitance that is necessary for driving of and detection by the vibration beam 3 can be optimized by adjusting these dimensions properly.
Then, as shown in FIG. 10, p++ polysilicon 14 is formed over the entire surface and patterned to form etching liquid introduction holes E for sacrificial layer etching. The p++ polysilicon layer 14 will later serve as interconnections to lead from the shell 4 to an electrode. The interconnections can be formed using p++/p+ single crystal silicon or by diffusing an impurity into the silicon substrate 1 before the selective epitaxial growth. It is advantageous that the interconnections be formed by a method that minimizes parasitic capacitances between the silicon substrate 1 and the interconnections.
Subsequently, as shown in FIG. 11, hydrofluoric acid is introduced through the etching liquid introduction holes E, whereby part of the silicon nitride film 13 and part of the silicon oxide film 10b are removed. In the connecting portions to the silicon substrate 1, the silicon nitride film 13 serves as an etching stop layer because of its low etching rate.
Then, as shown in FIG. 12, the p+ single crystal silicon layer 11 is removed by an alkaline solution (e.g., hydrazine, KOH, or TMAH). Neither the p++ single crystal silicon layer 12a nor the p++ polysilicon 14 is etched because of high impurity concentrations. During the etching with the alkaline solution, the n-type silicon substrate 1 can be protected from the etching by applying a voltage of 1 to 2 V to it. The etching in the longitudinal direction of the vibration beam 3 is stopped utilizing the fact the etching rate of single crystal silicon in the <111> direction is low.
Then, as shown in FIG. 13, sealing members 15 (made of SiO2, glass, or the like formed by sputtering) are formed by sputtering, evaporation, CVD, epitaxial growth, or the like, whereby the etching liquid introduction holes E are closed and a minute vacuum room 5 is formed. The electrical insulation between the shell 4 and the vibration beam 3 can be made more stable by, for example, forming silicon oxide films on the surfaces of the vibration beam 3 and the inside surfaces of the vacuum room 5 by thermal oxidation before this step. In this case, the sealing members can be made of a conductive material.
Then, as shown in FIG. 14, electrical interconnections which lead from the vibration beam 3 and the shell 4 are formed by patterning the p++ polysilicon layer 14 and bonding pad electrodes are formed.
Then, as shown in FIG. 15, a diaphragm is formed by thinning the silicon substrate 1 from the back side.
FIG. 16A is a plan view showing a state that electrical interconnections 20 have been formed so as to be connected to the vibration beam 3 and the shell 4 by patterning the p++ polysilicon layer 14 and bonding Al electrodes 21 have also been formed.
FIG. 16B is a circuit diagram of a resonant transducer including the assembly of FIG. 6. In FIG. 16B, Vb denotes a bias voltage (constant voltage); Vi, a drive voltage (AC voltage); R1 and R2, wiring resistances; and R3, a substrate resistance. C1 denotes a capacitance between the vibration beam 3 and the shell 4, C2 designates a parasitic resistance, and C3 and C4 stand for capacitances between the substrate 1 and the interconnections 20. The values of these parameters are determined by the interconnection forming method, the patterns of the interconnections 20, and other factors. These factors should be determined so as to minimize the values of these parameters.
Referring to FIG. 16B, if the capacitance C1 between the vibration beam 3 and the shell 4 is constant, the amplitude of an output current is proportional to (C1+C2)·Vi·ω, where ω is the frequency of the drive voltage Vi. On the other hand, when the capacitance C1 is resonating at the frequency ω, a current whose amplitude is approximately proportional to ΔC1·Vb·ω is added, where ΔC1 is a variation component of C1 due to the resonance. A resonant frequency is detected using this current change.
Incidentally, if no initial tensile strain is imparted to the vibration beam 3 even while no measurement pressure Pm exists, the vibration beam 3 may be buckled by a measurement pressure Pm to cause a state that a measurement cannot be performed. Furthermore, the sensitivity varies unduly unless the variation of this initial tensile strain is controlled. This will be described below.
FIG. 17 shows the covalent bond radii Ri of various impurities and their ratios to the covalent bond radius RSi of silicon.
As seen from FIG. 17, the covalent bond radii Ri of phosphorus (P) and boron (B) are 1.10 Å and 0.88 Å, respectively, which are shorter than the covalent bond radius RSi of silicon (Si) which is 1.17 Å. Therefore, when boron or phosphorus is implanted into single crystal silicon, tensile strain occurs in the implanted portion. In the case of boron, the strain is approximately equal to 4×10−4ε (=2×10−3/5.431) because the lattice constant variation of boron is 2×10−3 Å when its concentration is 1020 cm−3 and the lattice constant of silicon is 5.431 Å.
Strain that is larger than 4×10−4ε can be produced in the following manner. For example, if boron is implanted at a concentration of 2×1020 cm−3 which is two times the above value, initial tensile strain of 8×10−4ε occurs in proportion to the implantation amount. Therefore, an arbitrary initial tensile strain can be obtained by implanting boron at a proper concentration.
Based on the above principle, the vibration beam 3 shown in FIG. 6 is given initial tensile strain. The strain that is given to the vibration beam 3 can be made smaller than 4×10−4ε by increasing the boron concentration in the n-type silicon substrate 1 or lowering the boron concentration in the vibration beam 3 by causing boron atoms in the vicinity of the surfaces of the vibration beam 3 to segregate in an oxide by oxidizing the vibration beam 3 and then removing the oxide film with BHF. Almost no strain occurs when the boron concentration is equal to about 1017 cm−3 (see, for example, JP-A-01-299428 and JP-A-2005-037309).
The above-described conventional resonant transducer has the following problems. Since the vibration beam 3 vibrates perpendicularly to the substrate 1 and the vibration beam 3, an excitation electrode, and a vibration detection electrode have a layered structure, a manufacturing process requires many processing steps. Since selective etching utilizing an impurity concentration difference is performed in forming the vibration beam 3, the boron concentration should be set higher than 4×1019 cm−3.
In the epitaxial method, because of occurrence of defects and dislocations in the vibration beam 3, it is difficult to accurately control the tensile strain of the vibration beam 3 and the vibration beam 3 cannot be given high tensile strain when it is thick. Furthermore, a wafer may be warped by tensile film stress that is caused by the impurity. As such, in the epitaxial method, a vibration beam 3 that is thick and is given high tensile strain cannot be formed. On the other hand, to form a vibration beam 3 that is thick and is given high tensile strain by a related-art diffusion method, the diffusion length needs to be increased, resulting in a long diffusion time. For example, high tensile strain that is higher than 300με cannot be attained.
Furthermore, in the epitaxial method, a vibration beam 3 that is thick in the height direction cannot be formed because the tensile strain decreases in the case where it is thicker than about 5 μm.